1. Field of the Invention
This invention relates generally to machine-executed methods for managing a mount request in an automated data storage library and, more specifically, to a method for efficient selection of demount candidates from among a plurality of fully-occupied peripheral storage devices.
2. Discussion of Related Art
Modern computer systems require ever-increasing data storage capacities. A larger storage capacity often increases the random data access time, reducing computer system efficiency. Data access time can be improved by introducing a hierarchical data store organization. The improvement arises from an organization employing a small fast data store at the highest or first storage level and slower stores of increasing size and access time at lower data storage levels in the hierarchy in a manner well-known in the art. Such hierarchical organization is useful for optimizing the capacity, speed, cost and expendability of a data store and the associated computer system.
Modern computer data stores require very high capacity elements at the lowest level of the data store hierarchy. The Direct Access Storage Device (DASD) offers medium capacity and access speed but is limited by its fixed capacity and is unsuitable for archival storage operations involving a flow of storage media through the store. The automated data storage library offers archival storage capability and unlimited expandability in exchange for reduced access speeds arising from media handling delays. An automated data storage library includes provisions for storing a large number of data storage media (storage "bins") and one or more internal Peripheral Data Storage Devices (PDSDs), each for reading and writing on a single Data Storage Medium (DSM) mounted thereon. At least one robotic DSM handler ("picker") is also included in the library system.
Practitioners in the art have suggested several improvements to automated data storage library systems intended for use in hierarchical data stores. These at first involved tape cartridge library systems and, more recently, optical disk libraries. The optical disk was first limited to a write-once medium, thereby providing a permanent read-only data store. Later improvements in optical disk technology have introduced the rewritable optical disk DSM. Thus, modern automated data storage libraries now offer similar performance options with either tape cartridges or rewritable optical disks.
Automated library systems can be indefinitely expanded by adding new DSMs to overcome capacity limitations but they also delay data access when switching DSMs at each of the several internal PDSDs. These delays arise from the DSM management overhead in the library, including delays introduced by movement of DSMs from storage bins to PDSDs and back again. Some systems provide only a single picker to service several PDSD. Thus, under many circumstances, access to data is slowed by the delay between an access command from the upper data store hierarchy and the loading of the necessary cartridge or disk on a PDSDs. This is less of a problem for sequential data access patterns than for the random data access patterns that are serviced by the DASD stores in the hierarchy.
Several automated data storage libraries are known in the art. IBM Corporation introduced the 3850 Mass Storage Subsystem for the storage and retrieval of magnetic tape modules in the 1970's. More recently, several firms have introduced automated data storage libraries for magnetic tape cartridges and optical disks. The picker mechanisms of these libraries include one or more grippers, each gripper capable of handling one DSM at a time. IBM has also introduced the 9246 Optical Library Unit, which employs a two-gripper picker.
Normally, a single DSM or data volume is demounted from a PDSD immediately after the access to it ceases or upon request for access to a new DSM. The procedure for selecting a PDSD for demounting in response to a request to mount a new DSM is critical to the overall data access efficiency of the automated library system. This decision process must consider the possibility of additional accesses to the DSMs already mounted in place on the PDSDs. If the system permits a mounted DSM to be demounted prematurely, significant unnecessary data access delays can result from repeated remounting and demounting ("churning") of DSMs. The character of this problem depends on whether the data access request stream is primarily sequential (e.g., batch transfers ) or random (such as experienced in a system intended to emulate a DASD store).
Practitioners in the art have proposed several improvements to automated library systems to better balance capacity with speed. For instance, in U.S. Pat. No. 4,864,511, Michael E. Moy et al disclose a storage and retrieval subsystem employing an automated tape cartridge library having a split controller architecture that permits the library management controller to operate transparently to the host computer, thereby eliminating the host computer processing overhead associated with picker control.
In U.S. Pat. No. 4,864,438, Frederick G. Munro discloses an apparatus for managing tape cartridge movements in an automated cartridge library system that permits interconnection of two or more modular libraries so that tape cartridges can be interchanged among the modular libraries. Munro teaches a path selection system where a transit path through the multiple module library is reserved between the "home" point and "destination" point of a single tape cartridge. Once the path is selected, all intervening pass-through port slots are reserved in each library module along the path to ensure their availability for tape cartridge transportation. However, Munro does not consider the problem of efficiently resolving reservation conflicts involving the paths of several tape cartridges, other than by a simple First-In-First-Out (FIFO) priority scheme.
In U.S. Pat. No. 4,987,533, Connie M. Clark et al disclose a method for managing a storage hierarchy that includes an automated optical disk library. Clark et al teach a Least Recently Mounted (LRM) destaging decision method for destaging optical disks to an external manually-operated shelf storage. Clark et al suggest that, in an automated library system, the LRM technique is superior to the least recently used (LRU) technique known in the art for hierarchical store caching systems.
Christopher J. Monahan et al disclose a method for servicing a mount request for an automated data storage library in U.S. Pat. No. 5,121,483, which is entirely incorporated herein by this reference. Monahan et al teach a "virtual medium" method that permits the temporary demount of an active DSM that is subsequently remounted on any subsequently available PDSD. They also teach a preemptive demounting procedure that selects for preemptive demounting the LRM member of an "eligible subset" of all occupied PDSDs. Their "eligible subset" consists of all occupied PDSDs that are inactive longer than a predetermined time interval. However, Monahan et al neither consider nor suggest methods for dynamically optimizing their mount request servicing procedure in response to changes in the character of the data access request stream from the upper store hierarchy.
Modern large and midrange DASD stores are physically mounted at all times. Host operating systems for DASD stores do not provide for mounting and demounting delays, unlike tape system handlers. Optical libraries represent a new storage family having performance and costs midway between the tape storage systems and the high performance DASD stores known in the art. The cost and performance requirements for the optical PDSDs used in automated optical library systems discourage adding more than a few such drives to a given product. This cost-to-performance decision may change as the relative data access loads for automated library applications increase, but the present issues familiar to practitioners in the art require the rapid integration of optical libraries into existing data management systems. Such libraries must present a familiar and supported system image to the user and the host processor. DASD stores represent the most flexible and broadly applicable of the storage family candidates that are already fully supported by host systems. Accordingly, an optical library that simulates a DASD store fulfills a strongly-felt need in the art. However, simulating the appearance of permanently-mounted media in a PDSD-constrained environment is complicated by the high cost of multiple optical PDSDs and the slow pickers used to move optical DSMs to and from the PDSDs.
The IBM 3995 model 151 Automatic Optical Disk Library is a read-write optical library and is designed to emulate a 3390 DASD. Certain features of the model 151 are described in detail in "IBM Storage Subsystem Enhancements", IBM Doc. No. GG24-3886-00, IBM International Technical Support Center, San Jose, July 1992, pp. 107-146. DASD-emulation in an automated data storage library system introduces new problems for which solutions are unknown in the art. The typical workload directed to a DASD-emulating optical library consists of several different data-request streams, each having different characteristics. But random access requests and sequential access request streams appear to dominate the workload.
Random request streams result in frequent PDSD demounts and mounts, which implies heavy picker use and consequent dominance or "hogging" of the picker by these random streams. This dominance decreases the overall library access efficiency. Sequential request streams fill one DSM before demount and mount operations and thus do not increase mounting activity but do imply extended PDSD residency times. If the library controller fails to distinguish between random and sequential data access activity, a sequentially-accessed optical DSM cartridge will likely be prematurely demounted in favor of a recent random access mounting request. Such churning increases picker workload and reduces access efficiency.
Accordingly, there is a clearly-felt need in the art for improved methods of effective mount management in an optical library intended to emulate a DASD store. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.